1. Field of Invention
The present invention relates to a semiconductor manufacturing process. More particularly, the present invention relates to an etching process that uses carbon monoxide (CO) and fluorobutane (C.sub.4 F.sub.8) during plasma enhanced oxide etching in order to increase the etching selectivity and to avoid the premature etching stop for the etching process.
2. Description of Related Art
Earlier plasma-enhanced method of etching employs fluoromethane (CF.sub.4), fluoroform (CHF.sub.3) and argon (Ar) as the etching gases. However, because insufficient polymers are generated by the etching gases to protect other layers in the semiconductor structure, subsequently, high-density plasma (HDP) oxide etching process using fluorobutane (C.sub.4 F.sub.8) and carbon monoxide (CO) as the etching gases is developed. Under the high-density plasma oxide etching operation, the operating pressure is smaller than about 50 milliTorr (mT), hence the by-product formed by the etching gases C.sub.4 F.sub.8 and CO can be easily pumped out. In addition, concentration of ions in the plasma used in the HDP process is very high. Therefore, although a larger amount of polymers is deposited during the etching operation, there will be no occurrence of the etching stop phenomenon. The etching stop phenomenon refers to a sudden slowing of the etching rate due to too much deposition of polymers blocking the plasma etching action.
However, in a plasma-enhanced mode of oxide etching, the operating pressure is higher than 200 mT, and is unsuitable for using etching gases C.sub.4 F.sub.8 and CO.
In a HDP oxide etching process, the gaseous fluorobutane (C.sub.4 F.sub.8) can serve as an etching gas and at the same time a source gas for forming the polymers. This is because gaseous carbon difluoride (CF.sub.2) can be ionized out from the gaseous C.sub.4 F.sub.8 to form polymers protecting other semiconductor layers and increasing its etching selectivity. Furthermore, free radicals of fluorine (F) can also be ionized out from the gaseous C.sub.4 F.sub.8 to act as the source etchant. On the other hand, carbon monoxide (CO) serves as a gaseous agent for trapping free radicals of fluorine. Therefore, in the etching process, the addition of carbon monoxide helps to lower the etching rate. In other words, there is a tug-of-war going on between etching away the oxide layer and depositing a polymer layer on the surface. Consequently, when the etching rate is higher than the polymer deposition rate, etching of the oxide layer will continue. On the other hand, when the polymer deposition rate is higher than the etching rate, the etching of the oxide layer will simply stop. Moreover, the polymer layer will tend to roughen the etched surface so that it becomes very difficult to measure thickness with a thermal wave monitor.
Furthermore, if the polymer by-product of the etching operation is not thoroughly removed before the deposition of metal to form a self-aligned silicide (Salicide) layer, the polymer will react with the metal atoms to form high resistance layers that can affect the reliability of the product.
FIG. 1 is a block diagram illustrating the steps in carrying out a first conventional etching process. First, in step 10 of FIG. 1, a substrate is provided. The substrate has, for example, an oxide layer and a silicon nitride layer or a polysilicon layer formed over the substrate. Then, the substrate is placed inside a reaction chamber to carry out etching step 12, wherein the gas pressure inside the reaction chamber is set to about 300 milliTorr and the electrical power set to about 1300 Watts. In the conventional process, a gaseous mixture containing CHF.sub.3 /CF.sub.4 /Ar is passed into the reaction chamber, wherein the gas flow rates of CHF.sub.3, CF.sub.4 and Ar are 30, 30 and 400 SCCM (Standard Cubic Centimeter Per Minute), respectively.
Table 1A and Table 1B below show the test results of using a first conventional etching process. The purpose of the experiment is to compare the etching selectivity between etching an oxide material versus other materials such as silicon nitride, doped polysilicon and photoresist. In the tables, PECVD represents plasma enhanced chemical vapor deposition method; hence, PECVD-Oxide and PECVD-Nitride represent plasma enhanced chemical deposited oxide layer and nitride layer respectively.
TABLE 1A ______________________________________ ETCHING RATE (.ANG./Min) ______________________________________ PECVD-Oxide 5882 PECVD-Nitride 4298 Doped Polysilicon 627 Photoresist 1132 ______________________________________
TABLE 1B ______________________________________ ETCHING SELECTIVITY ______________________________________ PECVD-Oxide/PECVD-Nitride 1.37 PECVD-Oxide/Doped Polysilicon 9.38 PECVD-Oxide/Photoresist 5.2 ______________________________________
As seen in Table 1A and Table 1B, the result shows a rather poor etching selectivity for a conventional etching process. The etching selectivity ratio between PECVD-Oxide and PECVD-Nitride is as low as 1.37.
Moreover, after the oxide layer above the substrate is etched to form a contact opening, polymers will also be deposited on the sidewalls and bottom of the opening. When these polymers are allowed to react with self-aligned silicide material in a subsequent process, a layer of reacted material will form on the sidewalls of the opening. Hence, it is critical to clean up the polymers before depositing metal; otherwise, the metal atoms can react with the polymers to form a high resistance layer affecting the reliability of the device.
FIG. 2 is a block diagram illustrating the steps in carrying out a second conventional etching process. First, as shown in step 20 of FIG. 2, a substrate is provided. An oxide layer having a thickness of about 4000 .ANG. formed by a thermal oxidation method is formed over the substrate, and a BPTEOS layer having a thickness of about 7100 .ANG. is also formed over the oxide layer. Next, the substrate is placed inside a reaction chamber, and then the gas pressure is set to about 200 mT and the power rating set to about 1500 Watts. Subsequently, a gaseous mixture of CF.sub.4 /C.sub.4 F.sub.8 /CO/Ar/N.sub.2 having gas flow rates of 20, 4, 200, 600 and 20 SCCM respectively is passed into the reaction chamber for about 120 seconds to carry out the main etching step 22. Thereafter, a first thickness monitoring of the BPTEOS layer is conducted. Next, under the same pressure setting, power setting and the same gaseous mixture and the same respective flow rates of individual gases, an over-etching step 24 is carried out for about 60 seconds. Thereafter, a second thickness monitoring of the thermally formed oxide layer is conducted. However, in the above conventional etching operation, because of the difficulties in ensuring a higher etching rate than a polymers deposition rate, it is quite easy for the etching to stop before the etching operation ends.
Table 2 below shows the test result of using the second conventional etching process. The purpose of Table 2 is to compare the change in etching rate between the main etching step and the over-etching step. In this second convention etching process, the etching rate of the BPTEOS layer for the first 120 seconds is about 3915 (.ANG./min), but when the etching is continued for another 60 seconds, the etching rate of the oxide layer in this second period becomes just 928 (.ANG./min).
TABLE 2 ______________________________________ First Thickness Second Thickness Monitoring Monitoring Before Etching (BPTEOS) (Thermal Oxide) ______________________________________ Thickness (.ANG.) 11153 3322 2393 Etching Rate 3915 928 (.ANG./min) ______________________________________
In light of the foregoing, there is a need to improve the above etching methods.